1. Field of the Invention
The present invention relates to a branching filter that is used in small mobile communication devices such as mobile cellular phones. In particular, the present invention relates to branching filters and the construction of branching filter packages that use a high Radio Frequency (RF) filter as an elastic Surface Acoustic Wave (SAW) resonator type filter (called a SAW filter below) to achieve increased miniaturization and high performance.
2. Description of the Related Art
In recent years, the development of small and light-weight mobile communication devices, typified by devices such as mobile cellular phones, is advancing rapidly. With such advances come the demand for further miniaturization and higher performance of branching filters that are used by such mobile communication devices. Devices like the SAW branching filter, which uses a SAW filter, has tremendous potential for achieving further miniaturization of the mobile communication devices. In addition, they are also required to have small insertion losses at the pass band and large attenuation at the attenuation band.
SAW branching filters used in conventional mobile communication terminal devices such as mobile cellular phones have been disclosed in Japanese Patent Application Laid-Open No. H6-97761. In this type of SAW branching filter, the impedance matching circuit located between the antenna terminal and the receiver terminal, and the receiver filter, are connected in a serial manner. In addition, the phase matching circuit located between the antenna terminal and the receiver terminal, and the receiver filter, are connected in a serial manner. Furthermore, both the transmitting filter and the receiving filter form a ladder type resonator filter by arranging the serial arm SAW resonator and the parallel arm SAW resonator.
Due to the fact that such filters possess different center frequencies, for the various filters used for transmitting and receiving, the insertion losses at the frequency pass band is smaller and the attenuation at the frequency damping band is large in comparison. In order to suppress the mutual interference between the transmitting filter and the receiving filter that have different characteristics, it is necessary to maintain insulation between the transmitting SAW branching filter and the receiving branching filter described above. For example, the transmitting filter and the receiving filter are formed on different piezo electric base boards.
When the two filters are housed in the same filter package, it is necessary to decide whether to house the two filters in separate cavities that exist in the package (where the transmitting and receiving filters are separated using the wall of the package), or to house the transmitting and receiving filters in one cavity of the package while maintaining proper isolation between the two filters by keeping a certain distance between the two filters. In addition, the phase matching circuit and the impedance matching circuit are also formed together with the transmitting and receiving filters on the same package.